Units of Measurement

 Measurement is used to measure quantities.  Quantity is something that has magnitude, size, or amount (volume).  In the late 18 th century, scientists used the metric system. The metric system is a precursor to the SI System.  Scientists all over the world use a single measurement system called Le Systeme International d’Units, abbreviated SI.

 This system was presented in 1960 by a General Conference of Weights and Measures and has both base units and derived units.  It is based on the metre-kilogram-second system and replaces both the foot-pound- second system and the centimetre-gram- second system.  The International System of units has been developed and agreed internationally in the interests of world health.

 It overcomes languages barriers, enabling an exchange of health information within a country and between nations to be made without the misunderstandings which arise when each country, or even a separate hospital within a country, uses its own units of measurements for reporting tests. It is therefore important for health authorities and laboratories to adopt the new system of units. They are not difficult and are already being used in most medical publications and journals, by the manufacturers of chemicals and reagents, and by most scientific and medical organizations.  There are seven SI base units, i.e. metre, kilogram, second, mole, ampere, Kelvin and candela.

 The most common base units that we will study include: Quantity Quantity Symbol Unit name Unit abbreviation Length l meterm Mass m kilogramkg Time t seconds Temperature T KelvinK

To enable the measurement of quantities larger or smaller than the base units or derived units, the SI Units system also includes a set of prefixes. The use of a prefix makes a unit larger or smaller. The ranges of SI unit prefixes are listed in the tables 1 and 2

Divided byFunctionSymbolPrefix ddeci ccenti mmilli µmicro nnano ppico ffemto

Multiply byFunctionSymbolPrefix dadeca hhector kkilo Mmega GGiga Ttera Ppeta

 The SI standard unit for length is the meter.  A distance of 1m is about the width of an average doorway.  To express longer distances, the kilometer (km) is used. One kilometer is equal to 1000 m.  To express shorter distances, the centimeter (cm) is used. One centimeter is equal to 1/100 of a meter.  Length can be measured using a meter stick or rulers.

 Mass is a measure of the quantity of matter. The standard unit for mass is the kilogram (kg).  The gram (g), which is 1/1000 of a kg is used for measuring masses of small objects. Mass is typically measured using a balance.  Mass is different from weight. Weight is a measure of the gravitational pull on matter (Newton). The weight of an object increases as gravity acts on it.

 The standard unit of measurement for time is the second (s).  Time can be measured using stop watches, clocks, count down timers, and other time pieces.  Larger amounts of time are measured in minutes and hours.  There are 60 seconds in one minute. There are 60 minutes in one hour. Given that there are 24 hours in one day, how many seconds are there in one day?

 The standard unit of measurement for temperature is degrees Kelvin (K).  Temperature can also be measured in degrees Celsius ( °C) and degrees Fahrenheit ( °F).  To convert degrees Celsius ( °C) to degrees Fahrenheit ( °F) multiply by 1.8 and then add 32. To convert degrees Fahrenheit to degrees Celsius, subtract 32 and then divide by 1.8  ° K= °C  Temperature is measured using a thermometer. (measures the degree of heat or coolness)

 Derived units are combinations of base units. They are produced by multiplying or dividing standard units. The derived units we will study include: Quantity Quantity symbol Unit Unit abbreviation AreaAsquare meterm2m2 VolumeVcubic meterm3m3 DensityD kilograms per cubic meter kg/m 3

 Area is length times the width. It is expressed as square meters.  Area can also be expressed as cubic centimeters.  What is the area of a rectangle that has an a length of 6 cm and a width of 16 cm? 16 cm  1 6 cm  11111cmam

 Volume is the amount of space occupied by an object. The derived SI unit for volume is cubic meters. The cubic meter is rather large, so a more common unit of cubic centimeters is more commonly used.  Non-SI units are also used to measure volume such as the milliliter (mL) and the liter (L), which is 1000 cm 3. There are 1000 mL in 1 L.  Beakers, flasks, and graduated cylinders are often used to determine the volume of liquids.

 Density is the ratio of mass to volume, or mass divided by volume. It can be written: density=mass/volume or D=m/V Density is a characteristic physical property of a substance that does not depend on the size of the sample. As the mass of an object increases, its volume increases.